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What is DSD audio– A Simple Explanation

Paul McGowen, of PS Audio, wrote a wonderful short explanation of what DSD audio is and how it differs from PCM audio. Here is an excerpt from his post on DSD audio or 1-bit audio and why it sounds closer to analog sound than PCM.

1-bit audio is simple to understand in concept. There are no samples, there are no words, there is no code. Instead there is a continuous streaming “train” of single identical bits that are either on or off. The more bits that are on, the higher the eventual output voltage becomes. The more bits that are off, the lower the eventual output voltage. We refer to this type of scheme as Pulse Density Modulation because when you have a greater number of on bits it appears as more densely populated. Here’s a picture that will help you visualize a 1-bit system.

Note the blue areas are on and the white areas are off. Also note the periodicity between single bits is identical. The red sine wave overlaid on this image shows the results of more bits or fewer bits. Where there are no on bits (all white) the sine wave is at its lowest point – lots of on bits and it’s at its highest point.

The speed of the bits is 64 times the sample rate of a CD and some DSD schemes run at 128 times faster than a CD.

Here’s the interesting part of this: if you take a DSD stream and run it through a simple analog lowpass filter to smooth out the on/off transitions, you get music! This is amazing considering that if you do the same with PCM you get only noise.